It is important that aircraft avionics systems be designed to withstand the large changes in voltage, current and magnetic field when an aircraft is subjected to a lightning strike. Adequately protected avionic systems are even more important in the new generation of fly-by-wire aircraft where control of aircraft systems is accomplished entirely by electrical signals.
In the United States, the Federal Aviation Administration (FAA) has established minimum standards for protecting aircraft avionics systems from lightning. Previously, in order to test the avionics systems, a lightning strike on the aircraft was simulated by a test set which generated a single burst of either one Mhz or ten Mhz damped sinusoidal high energy pulses. More recent studies of aircraft lightning strikes have indicated that an aircraft lightning strike can generate multiple bursts of high energy. In response to a recommendation by the SAE Committee Draft Advisory Circular on "Protection of Aircraft Electrical/Electronic Systems Against the Indirect Effects of Lightning", the FAA has required that avionics testing be accomplished in a multiple burst environment.
It is recognized that only a fraction of the total energy of a lightning strike will appear at the avionics boxes inside the aircraft. This is because aircraft fuselage and related components provide a significant amount of shielding. By properly routing the system cables, the H field coupling between the cables and the aircraft frame can be significantly reduced. Further protection of the avionics systems can be provided through proper shielding of the cables and boxes.
Using established techniques, the "worst case" voltage and current to appear at the avionics boxes can be determined. This worst case signal provides a standard for simulation by a lightning strike simulator. It has been determined that the optimum simulation of a lightning strike is achieved by a multiple burst signal having a number of individual pulses per burst wherein the intervals between pulses as well as the intervals between bursts are random in nature.
Previous attempts to provide a multiple burst simulator have included a so-called "chatter system" which uses a coil and a commutator. The chatter system has proved to be unsatisfactory because, among other reasons, it fails to generate sufficient energy to accurately simulate a lightning strike. In addition, the pulses generated by the chatter system cannot be accurately duplicated thereby making it difficult to detect and analyze equipment disturbances caused by these pulses.
Other conventional systems include a random signal generator described in U.S. Pat. No. 2,974,198 by McLeod which generates independent signals of random occurrence and length for use in testing transmission systems. The generator uses a number of parallel pulse generating circuits which are responsive to filtered signals from a noise source.
In addition, Russian Patent No. 312253 discloses a random time interval generator which includes a random pulse generator which is connected downstream through a shift register and a number of coincidence circuits to respective Pulse generators.
Furthermore, RCA Technical Note #818, "High Frequency SCR Power Generator", discusses a high power generator which uses a number of SCR's which are switched sequentially to generate a continuous wave signal.